Apparatus for rapping electrodes in electrical precipitators



May 15, 1951 R. A. DAWSON 52,

APPARATUS FOR RAPPING ELECTRODES IN 1 ELECTRICAL PRECIPITATORS Filed Oct. 26, 1948 2 Sheets-Sheet 2 INVENTOR. 7 BY A OBE/er A, 3/1 W50/v ATHJORNEYS Patented May 15, 1951 APPARATUS FOR RAPPIN G ELE CTRODES IN ELECTRICAL PRECIPITATORS Robert A. Dawson, Montreal, Quebec, Canada, assignor to Western Precipitation Corporation, Los Angeles, Calif a corporation of California Application October 26, 1948, Serial No. 56,654

The present invention relates generally to electrlcal precipitators for removing suspended particles from a stream of gas, and more particularly to improvements in pneumatic rapping devices for cleaning electrodes in the precipitator of accumulations of dust.

In electrical precipitators of this type, a stream of gas carrying particles in, suspension passes through one or more electrostatic fields in which the suspended particles receive a charge and are then moved, under the influence of an electric field; toward a collecting electrode. When the collected material is a dry solid, it ordinarily accumulates upon the surface of the collecting electrode, and it then becomes necessary to cleanthe electrode periodically in order to maintain the efilciency of the precipitator. Under these circumstances, clearing an electrode can be accomplished' by delivering a series of sharp blows to the electrode to jar. or shake loose accumulated material which then, of its own weight, falls downwardly beneath the electrode and is usually collectedin the bottom of the shell or housing of the precipitator which is formed as a hopper for this purpose.

Although the bulk of the suspended material ordinarily accumulates upon the collecting electrode, it is common that a certain proportion of the material accumulates upon the high tension electrode; and this condition is particularly true when the precipitator is of a type having a plurality' of electric fields. A conventional and formerly widely used method of rapping an electrode is to deliver blows to the electrode assembly by a manually operated mechanism. This method has very distinct disadvantages which have resulted in the development of automatically operated mechanisms for rappers which are adapted to actuate the rapping mechanism at fixe'diintervals of time. Some of these automatic mechanisms have involved complicated timing valves cr other timing devices which, are expensive to construct and to maintain, and in some cases are not sufliciently reliable in operation to be entirely satisfactory.

These considerations have led to the development of a seli-timing pneumatic rapper which is entirely se1f-contained and dispenses with external timing valves. Rappers of this type are disclosed in the co-pending applications of Floyd paratus for Introducing Air to an Insulated Electrode Structure in an Electrical Precipitator (now Patent 2,526,715, dated October 24, 1950). Reference may be made to these co-pending applications for additional details relative to installation, construction, and operation of selftiming pneumatic rappers of the type herein disclosed. Certain features and the method of op:

eration disclosed herein are claimed in these earlier applications.

A pneumatic rapping mechanism of this character comprises generally a cylinder attached to the electrode assembly with a freely movable pistoh therein, the cylinder being provided with an inlet ior fluid under pressure from a fluid res'erwall, cooperating valve faces being placed on,

these two parts. Some diiiiculty has been encountered in obtaining air-tight engagement ofv these surfaces, and especially to maintain it aft er extensive operation because wear on the valve faces permits air to leak past them into the cylinder; In. this type of rapper the hammer falls under gravity" after the upward rapping blow is struck; and wear of the sealing surfaces takes place at the, end of the return stroke. prience has indicated that the pressure-tight, engagement of the surfaces can be greatly extended in life if these surfacesare not subjected to impact from the falling hammer; and. as ,a consequence operation of the hammer. is improved and maintenance greatly decreased.

Hence it becomes a general object of my invention to insure regular and uniform operation of self-timing rappers for electrodes of electrical precipitators.

It is another object of my invention to provide means, forcontrolling the flow of fluid under pressure into the cylinder which is simple in cone struction, yet effective for the intended purpose andable to operate over an extended period of time without maintenance or replacement. Simplicity of construction has the added advantages that'the device issimple and easy to maintain and repair.

These and other objects of my invention are attained in a rapping device of the character described above by providing a sliding-type valve the sliding valve means, which can be moved to I open position by a predetermined fluid pressure in the reservoir. The valve means is capable of limited movement in response to such pressure without opening the port. When the upward force applied to the valve by gas pressure within the reservoir exceeds the Weight of the piston, the valve moves to the open position, following which gas pressure is applied to the full area of the underside of the piston and the piston lifts rapidly, delivering a solid rapping blow of a type peculiarly effective in removing adhering dust deposits on the electrode. After the piston has lifted to a point where the exhaust ports are uncovered, the pressure within the cylinder drops rapidly, allowing the piston to return to a rest position. The hammer is designed so that the. force of the falling piston is not delivered to the valve assembly, but the energy is dissipated by engaging the cylinder. When in its rest position,

the piston is supported independently of the tion, the piston has a flange at its upper end which is adapted to seat upon the upper end of the open cylinder. In this form, the piston does not engage the'bottom of the cylinder but is sufficiently long to hold the valve closed when the peripheral flange is seated on the cylinder wall.

In a modified form of my invention, the lower end of the piston is recessed so that the piston is supported at points around its periphery on the bottom Wall of the cylinder and at a position centrally of the piston engages the valve to hold it closed when the piston is in the rest position.

.The fluid reservoir has an inlet of relatively small diameter so that an appreciable length of time, say several seconds, is required to completely fill the reservoir and bring its pressure up to the pressure of the fluid supply line. Ordinarily the predetermined pressure at which the valve into the cylinder opens is somewhat lower than this maximum pressure, but it still takes several seconds for the pressure within the reservoir to build up to this predetermined operating value. Control of this interval pf time or of the rate at which the reservoir fills determines the frequency of blows delivered by the piston for any given installation.

How the above objects and advantages of my invention, as well as others not particularly mentioned, are attained will be more readily understood by reference to the following description and to the annexed drawings, in which:

' Fig. 1 is a fragmentary elevation of the upper end of a collecting electrode assembly showing a self-timing pneumatic rapper mounted on the assembly;

2-2 of Fig. l; r

' Fig. 3 is a further enlarged vertical section of a 4 ing the pneumatic rapper only, and with the piston in the rest position;

Figs. 4, 5, and 6 are fragmentary sections similar to Fig. 3, showing successive positions of the valve and piston; and

Fig. 7 is a combined elevation and partial section through a modified form of pneumatic rapper showing the cylinder with a closed end.

In order to fully illustrate my invention, there is shown in Fig. l a portion of the discharge electrode assembly of an electrical precipitator which is typical of industrial precipitators for collecting.

dry solids suspended in a stream of gas. As the construction and arrangement of the various parts of electrical precipitators are Well known to persons skilled in the art, it is believed that a description of the details of the precipitator itself is unnecessary here except for a typical illustration of how the pneumatic rapping device is attached to an electrode assembly. Therefore, it

will be understood that my invention is in no way limited to the particular electrode construction shown herein, but may be applied equally well to collecting or discharge electrodes of other constructions than'shown herein. Various modifications of the location and attachment of the rapper are within the scope of persons skilled in the art.

Fig. 1 illustrates the upper portion of a hightension electrode assembly viewed from one end of a precipitator, so that the direction of gas flow is perpendicular to the plane of the drawing. The assembly comprises a pair of upright angles It which are interconnected at their upper ends by horizontal channels l2 and it. The lower ends of angles it are likewise connected by a horizontal member similar to channel E? but not shown in the drawings. Diagonal bracing i5 is added to give desired rigidity to the assembly. These structural members compose a framework which extends transversely of the gas stream and is suspended by hangers it from suitable insulated supports, not shown; Hangers it may be any type of structural member, but are preferably hollow tubes in order that at least one of the hangers may be used as a means for introducing the compressed air line. The details of this construction are illustrated and claimed in co-pending application Ser. No. 782,076 (now Patent 2,526,715)

referred to above. 7

Horizontal channel l2 supports one end of a plurality of pipes if; in a manner shown best in Fig. 2. Channel 12 has a plurality of pins 2!! which extend a short distance above the top edge of the channel and serve as individual supports for horizontallyextending pipes 53. The pipes are drilled on one side only near the end in order that a pin 25 can pass through the wall of the pipe and rest against the inside face of the upper Wall of the pipe, as shown in Fig. 2. Each pipe I8 supports a plurality of individual high-tension electrodes 2| which are spaced at intervals along the pipe. Electrodes 2! are wires or twisted membersof relatively small diameter suspended from a pipe is, as shown in Fig. 2.

Horizontal channel id is located directly below the upper channel I? the proper distance so that the rapping device, indicated generally at 23, can be mounted upon the lower channel and yet strike against an anvil plate 25 mounted on the under side of channel l2. Although the rapper 24 can'be fastened directly to channel i l, it is preferred that it be rigidly mounted, as by bolts or other fastening means, toplate. 26 which is fastened to the top of the channel bywelding rapper viewed at to the plane of Fig. 2 show- 76 and reinforced by bracket 2]. Anvil plate 25 is similarly fastened to the under side of channel l2 and reinforced by brackets 28.

Rapper 24 comprises cylinder 30 within which piston (ii is freely movable relative to the cylinder. Cylinder 3ft is open at its upper end, and the top end of the cylinder provides a continuous annular seat 380. upon which peripheral flange am of piston Si is adapted to seat when the piston is lowered in the cylinder to the rest position shown in Fig. 3. Flange am is located at the upper end of the piston and is larger than the internal diameter of cylinder 30 so that engagement of the flange with the cylinder wall limits downward movement of the piston relative to the cylinder. Thus the piston is supported on the cylinder when in the normal or rest position.

Cylinder 3 is ordinarily located with its longitudinal axis extending upwardly, as shown in the drawings. This is done so that the piston is returned by gravity from the raised position at the end of the stroke, as shown in Fig. 2, to the rest position of Fig. 3. For other reasons which are discussed later, the cylinder and piston are substantially the same length so that the cylinder surrounds substantially all of the piston; but this general proportion of parts is also desirable in order to obtain a maximum amount of guiding action by the cylinder and also permit a reasonable length of stroke. Otherwise special guide means must be provided in order to insure the proper return of the piston into the cylinder. Hammer piston 3! is a loose sliding fit within the cylinder. The normal clearance between the piston and the cylinder wall is preferably approximately of an inch, but it may be changed keeping in mind that the operation of the hammer is modified appreciably if this clearance is rendered excessively large.

The side walls of cylinder 30 are provided with fluid exhaust or outlet ports 33 located just below the piston at the upper range of its travel. Ports 33 are here shown as being six in number; and their aggregate area is sufflciently large to insure rapid exhaust of fluid under pressure from the cylinder at the end of the piston stroke. Near the bottom of the cylinder walls are one or more bleeder ports 34 which are considerably smaller in diameter so that their aggregate area is not great enough to detract substantially from the force delivered to the piston by the fluid under pressure.

The lower end of cylinder 30 is closed by plate 35 which separates the interior of cylinder 30 from the interior of reservoir 3?, but has a bore 38 connecting the interior of cylinder 30 with reservoir 3?, through which bore fluid leaves the reservoir and enters the cylinder. Inlet port 38 is closed by valve means of the sliding type, here shown as being cylindrical sleeve 40 which is adapted to slide longitudinally within bore 38.

Valve member M3 is a hollow member closed at its upper end and open at its lower end. The side walls of the valve are pierced by a plurality of openings 4|. Two oppositely disposed openings il are elongated and through them passes pin 42 which is held in valve guide 43. Valve 48 is free to slide up and down within passage 38 and its movement is limited by pin 42, the range of movement being determined by the length of the elongated slots 4!. Valve 40 is a close sliding fit within inlet 38, sufficiently so that air under pressure does not leak out of reservoir 3'! into the upper cylinder when the valve is lowered within the inlet.

The valve assembly is held in place by threaded 6 engagement of valve guide 43 with stationary nut 45 which is welded or otherwise attached vto the under side of plate 36. A metallic washer 46 is preferably used to insure an air-tight fit between the guide 43 and plate 36.

Beneath cylinder 30 and plate 36 is located fluid pressure reservoir 31, which is conveniently made circular in cross section. The lowerside of reservoir 3! is closed by plate 41 which forms a base by which rapper 24 is mounted on plate 26, as shown in Fig. 2. At aconvenient point in a side wall, reservoir 31 is provided with a threaded plug 48 which has a smalldiameter bore 49 drilled longitudinally of it. Bore 49 provides an inlet orifice for fluid under pressure entering reservoir 3?, and is only a few hundredths of an inch in diameter in order to meter the flow 0f fluid into the reservoir. By providing the inlet orifice in a threaded plug as shown, plug 48 can be quickly replaced with another one in order to change the size of the orifice as desired.

On the exterior of reservoir 3'! and surrounding plug 48, is located a pipe union or coupling to which the fluid supply line 5! (shown in Fig. 1) is attached in order to supply compressed air or other suitable fiuid under pressure to the rapping device. It is within the scope of my invention to provide a filter 52 in the air supply line at some suitable point ahead of orifice 49 in order to strain out solid particles which might clog the orifice. This filter can be of fiberglass or any other suitable material conventionally used in filters.

When the rapper is on a high tension electrode assembly as in Fig. 1, it is preferable to havethe supply line 5i pass through a hanger [6, as described in greater detail'in the co-pending applications referred to above.

Although it is possible to vent exhaust ports 33 and bleeder ports 34 directly to the interior of the precipitator, it is ordinarily preferred not to do this because of the fact that dust carried by the gas stream and corrosive gases, if present, can enter the ports during periods of idleness v, and cause corrosion or increased wear of the its upper edge with plate .Ei i, which latter plate also seats on a shoulder on the exterior surface of cylinder 3b, as shown. The entire assembly is held rigidly in place by a plurality of bolts 55 which draw plates 54 and 36 together and force the bottom end of cylinder 39 into fluidtight engagement with its seat in plate 36. The annular space 56 between shell 53 and cylinder 39 is an exhaust chamber into which the cylinder exhausts spent fluid. This fluid leaves exhaust chamber 5&3 through an outlet port 51.

The enclosed exhaust chamber may provide adequate protection for the cylinder under some circumstances; but where it is desired'to provide additional protection, as is especially the case where the precipitator is treating corrosive gases, a pipe union or coupling 68 is fastened to the outside of housing 53 over outlet port 5?. An exhaust line 5| is then attached to coupling 60, as shown in Fig. 1. Exhaust line 60 passes upwardly through an electrode assembly hanger l6 and opens to the atmosphere outside the precipitator shell. In this way, gas from the stream within the precipitator is entirely excluded from the interior of rapper 24 and the working parts come into contact only with clean filtered air introduced through supply line '5 I.

I-laving described the construction of a preferred form of my invention, I shall now describe its operation. The air supply line is connected to a suitable source of compressed air, or any other fluid adapted to this same purpose, such as the reservoir of a conventional compressor unit. Flow of fluid through supply line 5! is continual during any rapping period during which hammer 24 is in operation; but since it is ordinarily sufficient in rapping to strike the electrode assembly with several blows and then wait for an interval of severa1 minutes before repeating this rapping, there may be provided in supply line 5! suitable valve means, not shown, for periodically shutting oh. the continuous flow of air through the supply line. For additional details of this apparatus, reference may be made to Viets application, Ser. No. 715,020 referred to above.

Compressed air from supply line 5! passes through metering orifice &9 into reservoir 3? from which it cannot escape when sleeve valve til is lowered and closes passage 33. Assuming the initial pressure within reservoir 3'! to be substantially'atmospheric, as it is after the end of each operating stroke of the hammer piston, air pressure within the reservoir builds up as air enters through port 59. Because the area of port ts is so very small, pressure within the reservoir builds up slowly; and, if no air were removed from the reservoir, it would require several seconds to completely fill the reservoir, that is to bring the prev sure within the reservoir up to the pressure of supply line 5!. The length of time required can obviously be controlled by controlling the size of metering orifice 49.

However, the hammer is designed to operate at some predetermined pressure within reservoir 3? which is lower than the pressure within supply line 5i. By way of typical example, if the supply line pressure is in the neighborhood of 75 to 80 pounds per square inch, the hammer is designed to operate at a reservoir pressure in the neighborhood of 40 to pounds per square inch. Metering orifice :39 is sufficiently small that it requires.

several seconds to fill up reservoir 3.? to this predetermined operating pressure from substantially atmospheric.

When the air pressure is the same on both sides of the valve it, it seats fully by gravity as shown in Fig. 3 and is supported in place by engagement with pin 22. As the first effect of increased pressure within reservoir 37, valve it rises slightly because of the increased pressure on its under side until it comes into engagement with hammer 3|, as shown in Fig. i. The valve can rise no further now because the weight of the hammer is suflicient to hold down the valve which is still in the closed position in which valve so prevents the flow of fluid from the reservoir through inlet 38 into the cylinder. When the predetermined operating pressure is reached in the reservoir, the pressure on the under side of valve Ail is sufficient to raise piston 35; and the piston moves upwardly relatively slowly until the valve reaches the position of Fig. 5 in which it has lifted sufiiciently that the upper end of openings ii are at the top edge of valve guide as. After the valve moves beyond this position so that openings 4! open into cylinder 36 above guide '43; air can flowfrom the reservoir through the hollow interior of The unit pressure against the end surface of' the piston is but slightly less than the predetermined pressure within the reservoir at which valve 40 opens because the additional space into which the compressed air has expanded is relatively small. Consequently the piston is driven upwardly with a relatively great force and strikes a sharp, vigorous blow against anvil 25, reaching at the end of its upward stroke the position shown in Fig. 2.

Exhaust ports 33 are located in the'walls of cylinder 30 slightly below the lower end of the piston when it reached the extreme upward range of travel shown in Fig. 2. In this way the ports are covered'during most or the forward or upward travel of the piston, but are fully uncovered to allow the escape of air from the cylinder when the piston reaches the end of its stroke;

When ports 33 are uncovered, the air within the cylinder rapidly exhausts through them into chamber 56, sufiiciently reducing the pressure within the cylinder that piston 38' is able to return, under th influence of gravity, to the rest position of Fig. 3.

Bleeder ports 3 are located near the bottom of the cylinder, but sufficiently far up on the walls that they remain fully covered by the piston until valve ii is iuliy opened (Fig. 6). Of course these ports cause some dimunition inpressure during the upward travel of the hammer, but they are sufficiently small that this decrease in pressure is not substantial as compared with what the pressure would be if ports 3 were not present. The primary function of ports 2% is to permit escape into chamber 56 of air trapped within the lower part of the cylinder on the downward stroke of the piston after it seals off eX- haust ports 33. For this purpose, the size of ports 34 is large enough that a slight cushioning effect is produced on the falling piston but at the same time the piston is permitted to drop at once to a fully seated position with its peripheral flange em in engagement with the seat 353a provided by the upper open end of the cylinder. It is possible to omit ports 3 but in this case it is necessary to provide somewhat greater clearance between the piston and the cylinder walls in order that the air trapped beneath the piston can flow by and escape either at the principal exhaust ports or at the upper end of the cylinder. This increased clearance is not preferred because it permits air to leak around the piston during the power stroke and also causes the cylinder walls to wear more rapidly.

When at the end of the power stroke air pressure is equalized between the cylinder and air reservoir 37, valve 463 returns to the closed position under the influence of gravity. If for any reason the valve does not previously close, the falling piston engages the raised valve and pushes it downwardly to a closed position, weight of the piston being sufficient to accomplish this even though the air pressure is higher in the reservoir than in the cylinder. Thus the piston operates not only to hold the valve closed but also, if necessary, to close the valve. In order to control the valve, the piston is sufiiciently long the.

axially that it engages the valve and holds it in a closed position even when seated at tile on the cylinder. It can also engage the valve when above the closed position and move it to that position.

In practice it has been found desirable that the hammer be operated at the rate of from twelve to twenty-four blows per minute, that is, the interval between power strokes of the piston is about three to five seconds. This frequency of blows is established by the relationship of various weights and areas and can be changed by any one of several factors, but it may be summarized as being determined by the rate at which air flows through orifice 9 to fill the reservoir and the force required to lift the valve and piston to open valve id. The frequency of operation being in herent in the design, it is maintained within narrow limits once the design has been established; and for this reason the rapping device has been termed selftiming. Ordinarily, it is unnecessary that rapping of the electrode assembly be continued over any extensive period of time, as it is only necessary to strike a suflicient number of blows to loosen accumulated deposits on the electrodes and then to cease rapping until deposits have accumulated again to an extent to impair efflciency of the electrodes, at which time rapping should again be instituted.

There is shown in Fig. '7 a modified form of my invention which is designed according to the same general principles and operates in the same fashion as described above, with the exceptions specifically pointed out below. In this form cylinder 63 is closed at both ends, the upper end of the cylinder being closed by plate 64 which also serves as an anvil against which piston 65 strikes when it is driven upwardly. The shock delivered to anvil plate 64 is transmitted through bolts 65 to I-beam 6'! upon which the rapper is mounted. This particular type of construction is especially adapted to mounting on collecting electrodes, not shown in the drawing but ordinarily suspended from I-beam 51. In order to transmit the shock of the blow with relatively high efliciency, bolts 66 are tightened up to place them under tension.

Piston 65 is made of uniform diameter throughout its length so that it is necessary to provide other means for supporting the weight of the piston independently of valve 40. In order to accomplish this support of the piston, the central portion of the piston is recessed, as shown in Fig. '7. This results in an annular surface or flange 65a, adjacent the periphery of the piston, which engages plate ll that closes the lower end of cylinder 53. The annular flange 65a need not be continuous, although it is preferable that it be continuous in order to provide the maximum amount of surface engaging end plate Ill.

The depth of the recess is such that when the piston is in the rest position of Fig. 7 with flange 65a engaging plate 2'8 to support the weight of the piston, none of the piston weight rests directly upon valve il However, in this position when valve M] is raised by pressure in reservoir 12 the valve engages the piston before it reaches an open position periinitting air to enter the cylinder from the reservoir. In this way the piston acts as previously described to hold the valve in a closed position to prevent flow of pressure fluid into the cylinder at less than a predetermined pressure in the reservoir at which the piston is raised and the valve opened.

The modification shown in Fig. 7 further illustrates a form of my invention in which exhaust chamber 56 and housing 53 have been omitted. This may be done when the rapper is located outside the stream of dust-laden gas, as is often the case with rappers on collecting electrodes.

It will be obvious from the foregoing that plate 'Jil could be centrally recessed to lower the valve from its present position sufliciently that the piston can have a flat lower end and still engage plate it! only over an annular area, as in Fig. '7. This is simply a transfer of the piston recess, or flange 65a, to the end plate of the cylinder.

Having described certain embodiments of my invention, it will be apparent that various changes in construction and arrangement of the parts may be made by persons skilled in the art without departing from the spirit and scope of my invention; and consequently I wish it understood that the foregoing description is considered as illustrative of, rather than limitative upon, the appended claims.

I claim:

1. In a pneumatic rapping device for an electrical precipitator, the combination comprising: a cylinder with its longitudinal axis extending upwardly, the cylinder having an outlet port in a side wall and an inlet port in the bottom wall communicating with a fluid pressure chamber; sliding valve means at the inlet port adapted to open the inlet port when in raised position and to close the inlet port when in lowered position; and a fluid-pressure actuated piston freely movable vertically relative to the cylinder and when in rest position adapted to close the outlet port and to hold the valve means in closed position to prevent inflow of fluid at less than a predetermined pressure of fluid in said chamber.

2. A pneumatic rapping device for an electrode assembly, comprising: a cylinder mounted on the electrode assembly with its longitudinal axis extending upwardly, the cylinder having an outlet port in a side Wall and an inlet port in the bottom wall; a fluid pressure reservoir beneath and communicating with the cylinder through the inlet port; a vertically movable valve member regulating fluid flow through the inlet port and movable between an upper position in which the inlet port is open and a lower position in which the inlet port is closed and capable of limited vertical movement without opening the inlet port; and a fluid-pressure actuated piston freely movable vertically relative to the cylinder and when in rest position adapted to close the outlet port and to hold the valve member in closed position to prevent inflow of fluid at less than a predetermined pressure of fluid in said reservoir.

3. In a pneumatic rapping device for an electrical precipitator, the colnibnation comprising: a cylinder with its longitudinal axis extending upwardly, the cylinder having an outlet port and an inlet port communicating with a fluid pressure chamber; a sleeve valve slidably mounted at the lower end of the cylinder to open and close the inlet port and movable upwardly to open position in response to fluid pressure in said chamber; and a fluid-pressure actuated piston freely movable vertically relative to the cylinder and when in lowered rest position adapted to hold the valve means in lowered closed position to prevent inflow of fluid at less than a predetermined pressure of fluid in said chamber, the piston having an area exposed to fluid pressure in the cylinder that is several times the area of 11 the sleeve valve'exposed to fluid under pressure tending to open the valve.

4. A rapping device as in claim 2 in which the reservoir has a fluid inlet of such small size that several seconds are required to raise fluid pressure therein to said predetermined value after each release of fluid from the reservoir.

5. In a pneumatic rapping device for an electrical precipitator, the combination comprising: a cylinder open at the upper end and having an inlet port at the other end; valve means at the lower end; of the cylinder adapted to open and closethe'inletport, said valve means comprisinga vertically movable valve member movable to a raised position in response to fluid pressure outside the cylinder'in which the valve member opens the port and projects into the cylinder ing the piston in its rest position; the piston being of suflicient length axially to engage and hold theivalve means in a lowered closed position when the piston is in said rest position; and a fluid reservoir communicating with the cylinder through the inlet port and having a fluid inlet of relatively small diameter so that when fluid under pressure is supplied continuously thereto several seconds are required to flll the reservoir.

6. In a pneumatic rapping device for an electrical precipitator, the combination comprising: a cylinder with its longitudinal axis extending up 7 wardly and having an inlet port formed by a bore located in the end wall of the cylinder and communicating with a fluid pressure chamber; a

sliding valve member in the port having a fluid passage opening at one end to a surface of the valve member that slidably engages the end wall, the bore wall covering the fluid passage to close the port when the valve member is lowered and uncovering the passage to open the port when the valve member is raised in response to pressure in said chamber, the valve member being capable of limited movement while keeping the port closed; and a fluid-pressure actuated piston freely movable relative to the cylinder, said cylinder and piston having cooperating means other than the valve member to support the weight of the piston in a rest position in which the piston engages the valve to hold it in a lowered closed position, the valve member being capable of downward movement to a position out of engagement with the piston in said rest position.

7. In an electrical precipitator, a rapping device for an electrode assembly, comprising: a cylinder mounted on the electrode assembly with its longitudinal axis extending upwardly, the cylinder having a pair of outlet ports and an inlet port communicating with a fluid pressure chamber; a valve member slidably mounted to open and. close the inlet port, and capable of limited sliding movement without opening the inlet port; and a fluid-pressure actuated piston 12 freely movable relative to the cylinder and when in rest position adapted to close said outlet ports and to hold the valve member closed to prevent.

inflow of fluid at less than a predetermined pressure of fluid in said chamber; one of said outlet ports being or" relatively large diameter to exhaust fluid rapidly from the cylinder and located near the upper end of the cylinder to be uncovered by the piston near the end of its upward stroke, and the other of said outlet ports being of relatively small diameter and located near the bottom of the cylinder to be uncovered after relatively short movement of the piston above the rest position.

8. 'In a pneumatic rapping device for an electrical precipitator, the combination comprising: a cylinder mounted with its longitudinal axis extending upwardly, the cylinder having an outlet port in a side wall and an inlet port in the bottom wall communicating with a fluid pressure chamber; a fixed valve guide in the bottom wall of the cylinder having a bore; a valve member slidable in the guide bore with an exterior surface in sealing engagement with the guide bore wall and provided with a fluid passage communicating with said pressure chamber and opening at one end to said exterior surface of the valve member to communicate with the cylinder when as valve member is'raised relative to the valve guide, the fluid passage being closed to the cylinder by the bore wall when the valve member is lowered in the valve guide; and a fluid-pressure actuated piston freely movable vertically relative to the cylinder and when in lowered position in the cylinder adapted to hold the valve member in lowered position to prevent inflow of fluid at less than a predetermined pressure of fluid in the pressure chamber. i

9. In a pneumatic rapping device for an electrical precipitator, the combination comprising: a cylinder mounted with its longitudinal axis extending upwardly,.the cylinder having an outlet port in a side wall and an inlet port formed by a bore in the bottom wall communicating with a fluid pressure chamber; sliding-type valve means comprising a valve member in said bore, the Valve member having a fluid passage in communication at one end with the fluid pressure chamber and being slidable in the bore between a raised open position in which the other end of the fluid passage communicates with the cylinder and a lowered closed position in which the bore wall closes said other end of the fluid passage; and a fluidpressure actuated piston freely movable vertically relative to the cylinder and when in lowered position in the cylinder adapted to hold the valve member in lowered position to prevent inflow of fluid at less than a predetermined pressure of fluid in the pressure chamber. 7

ROBERT A. DAWSON.

REFERENCES CITED The following references are of record in the file of this patent:

Hamilton Jan. 1, 1946 r 

